Close the small box, and go back to your inbox. Since the forwarding address is already set up, you'll receive a message shortly in the inbox called Gmail Confirmation - Send Mail as [forwarding address]. Open that message.

Click the confirmation link in the above message when it arrives.

That's enough to compose mail under the robertsstreet.org address. But there's a bit more work to be done.

Head back to your account settings (Settings » Accounts).

Now you'll see [at least] two addresses under 'Sent mail as'.

Look below those addresses at the option that begins 'When receiving a message', and click Reply from the same address the message was sent to.

Now when someone sends mail to the robertsstreet.org address and you reply to it, your reply address will also be the robertsstreet.org address. If, on the other hand, someone sends mail to the robertsstreet.org address your reply address will be your gmail address. Now there's just one more step, which is optional but which I strongly recommend.

Beside the gmail address it should read default. Beside the robertsstreet.org address there should be a link that reads make default. Click that link. Now all *new* mail you send from the account will come from the @robertsstreet.org account, while still working seamlessly with the original @gmail.com address!